1. Field of the Invention
The present invention relates to methods and apparatus for inspecting the surface of a substrate such as reticles, photomasks, wafers and the like (hereafter referred to generally as photomasks). More specifically, the present invention relates to an optical inspection system and method for scanning specimens at a high speed and with a high degree of sensitivity.
2. Description of the Related Art
Integrated circuits are produced using photolithographic processes, which employ photomasks or reticles and a light source to project a circuit image onto a silicon wafer. Photomask surface defects are highly undesirable and adversely affect the resulting circuits. Defects can result from, but not limited to, a portion of the pattern being absent from an area where it is intended to be present, a portion of the pattern being present in an area where it is not intended to be, chemical stains or residues from the photomask manufacturing processes which cause an unintended localized modification of the light transmission property of the photomask, particulate contaminates such as dust, resist flakes, skin flakes, erosion of the photolithographic pattern due to electrostatic discharge, artifacts in the photomask substrate such as pits, scratches, and striations, and localized light transmission errors in the substrate or pattern layer. Since it is inevitable that defects will occur, these defects are preferably located and repaired prior to use. Blank substrates can also be inspected for defects prior to patterning.
Methods and apparatus for detecting defects have been available. For example, inspection systems and methods utilizing laser light are available to scan the surface of substrates such as photomasks, reticles and wafers. These laser inspection systems and methods generally include a laser source for emitting a laser beam, optics for focusing the laser beam to a scanning spot on the surface of the substrate, a stage for providing translational travel, collection optics for collecting either transmitted and/or reflected light, detectors for detecting either the transmitted and/or reflected light, sampling the signals at precise intervals and using this information to construct a virtual image of the substrate being inspected. By way of example, representative laser inspection systems are described in U.S. Pat. No. 5,563,702 to Emery et al., U.S. Pat. No. 5,737,072 to Emery et al., U.S. Pat. No. 5,572,598 to Wihl et al., and U.S. Pat. No. 6,052,478 to Wihl et al., each of which are incorporated herein by reference.
Although such systems work well, ongoing work in the area seeks to improve existing designs to enable higher degrees of sensitivity, increase the ability to classify and quantify defects, and to allow faster scanning speeds and higher throughput. As the complexity of integrated circuits has increased, the demands on the inspection of the integrated circuits have also increased. Both the need for resolving smaller defects and for inspecting larger areas have resulted in greater magnification requirements and greater speed requirements.
Various methods exist to perform detailed inspections of patterned masks or reticles. One inspection method is a die-to-die comparison which uses transmitted light to compare either two adjacent dies or a die to the CAD database of that die. These comparison-type inspection systems are quite expensive because they rely on pixel-by-pixel comparison of all the dies and, by necessity, rely on highly accurate methods of alignment between the two dies used at any one time for the comparison. Apart from their high costs, this method of inspection is also unable to detect particles on opaque parts of the reticle which have the tendency to subsequently migrate to parts that are transparent and then cause a defect on the wafer. One such die-to-die comparison method of inspection is described in U.S. Pat. Nos. 4,247,203 and 4,579,455, both by Levy et al.
The second method for inspecting patterned masks is restricted to locating particulate matter on the mask. This method makes use of the fact that light scatters when it strikes a particle. Unfortunately, the edges of the pattern also cause scattering and for that reason these systems are unreliable for the detection of particles smaller than one micrometer. Such systems are described in a paper entitled “Automatic Inspection of Contaminates on Reticles” by Masataka Shiba et al., SPIE Vol. 470 Optical Microlithography III, pages 233–240 (1984).
A third example of a system for performing photomask inspection is disclosed in U.S. Pat. No. 5,563,702 to David G. Emery, issued Oct. 8, 1996. The system disclosed therein acquires reflected images, in addition to transmitted images, to locate defects associated with contaminants, particles, films, or other unwanted materials. Since this system locates defects without reference or comparison to a description or image of the desired photomask pattern, it may in certain circumstances not locate defects outside of known boundaries, such as where the transmitted and reflected images differ from an expected amount by a threshold amount.
Specific types of photomasks called APSMs, or Alternating Phase Shift Masks, are typically designed with thickness variations in the glass or quartz which induce phase shift transitions between adjacent regions during photolithography. Phase defects can exist which are unwanted thickness variations created by phase etch process errors, and have similar optical image signatures during inspection. Hence APSM phase defects are difficult to distinguish from design phase features using the system shown in U.S. Pat. No. 5,563,702. Phase defects cannot be detected by this system without producing false defect readings on phase shift design features where no defects actually exist. However, the transmitted and reflected imaging capabilities and defect detection operators of this system can be useful to determine the presence of phase defects if all detected phase features are properly compared and contrasted to reference photomask image data, as in a die-to-die or die-to-database system.
The use of reflected light in combination with transmitted light may improve detection of phase defects. The difficulty with using reflected light is managing image artifacts, such as the bright chrome halos resulting from the removal of the antireflective chrome layer during quartz etching of phase shifters. Bright chrome halos may have variable widths resulting from second write level registration tolerances with intra-plate variations. These variations are not observable when solely using transmitted light inspection techniques.
Thus, in general, phase feature signals captured with brightfield transmitted light may vary widely depending on mask and defect characteristics, and phase feature signals captured in reflected light may be stronger. On the other hand, use of reflected light can be problematic in the presence of image artifacts such as bright chrome halos. Therefore, die-to-die or die-to-database photomask inspection with transmitted and reflected light may benefit from signal-to-noise enhancements as well as an enhanced ability to discern phase shift features and phase defects.
During inspection, different light sources can drastically affect the quality of the information received in the presence of certain types of defects. The phase defect signal in brightfield transmitted light can be substantially less than that of a similarly sized chrome defect, thereby complicating the ability to inspect the mask. The phase defect's signal depends on a variety of factors, including the height or phase angle of the phase defect, the depth of the phase shifters, and inspection system optical parameters.
An alternate method employed for defect detection is an extension of the die-to-database comparison method, wherein the system compares two die with an identical design. The images employed may be either reflected or transmitted light energy images. One of the two die is called the reference die, and the other is called the test die. In this method, if the difference between the pixels of the reference die and the test die exceed a predetermined value, the pixel is marked defective. The transmitted threshold may be called δT, while the reflected threshold is δR. Performance of a defect detection algorithm may be viewed as a ΔT-ΔR plane as illustrated in FIG. 3. From FIG. 3, ΔT represents the transmitted image difference between the test and reference dies, while AR is the reflected image difference between the test and reference dies. Previous methods have performed transmitted image detection independent from reflected image detection, where a defective zone is separated from a non-defective zone using constant thresholds as shown in FIG. 3. These previous methods employed constant thresholds for determining and quantifying defects. Non-defective zones used in the presence of constant thresholds have been found to be excessively large, resulting in declaring defects when no true defects exist, requiring additional post processing. A smaller non-defective zone is preferable, as it translates to higher defect detectability. In other words, if the non-defective zone of FIG. 3 is small, a higher probability that a defect will be located exists. However, decreasing the non-defective zone excessively may result in too many anomalies being classified as defects. In actuality, many errors between dies exist, including errors between test and reference dies. If the threshold is too small, a large number of false or nuisance defects will develop, taking a great deal of time to inspect, decreasing overall throughput.
One type of defect identification system and method is presented in pending U.S. patent application Ser. No. 09/991,327, entitled “Advanced Phase Shift Inspection Method” to David G. Emery, filed on Nov. 9, 2001 and assigned to the assignee of the present invention. While the system disclosed therein provides for inspection of APSMs using transmitted and reflected light, certain defects may exist that may not be picked up by the Emery design. Further, the Emery design may require inspection of areas that are flagged as defective or potentially defective, potentially decreasing overall speed and throughput.
In view of the foregoing, there is a need for improved inspection techniques that provide for improved detectability while at the same time avoiding excess false and nuisance detections when scanning specific types of semiconductor specimens, such as APSM photomasks.